This invention relates to a facet tracking optical scanning systems and, more particularly, to a scan angle doubling optical system with a reflective binary diffractive optical element used as the rotating scanning element.
The prior art raster output scanning system 10 of FIG. 1 consists of a pre-polygon mirror optical section 12, a rotating polygon mirror scanning element 14 comprising a plurality of reflective facets 16, and a post-polygon mirror optical section 18 to correct for wobble of the rotating polygon mirror and to focus the beam along a scan line.
A light source 20, such as a laser, emits a coherent beam 22 of a single wavelength which is collimated by a collimating lens 24. The collimated beam then passes through a cross-scan cylindrical lens 26. The lens 26 may be cylindrical in the cross-scan plane and plano in the scan plane. Thus, the lens converges the cross-scan portion of the beam focusing it on a reflective facet 16 of the rotating polygon mirror 14 but allows the scan portion of the beam to remain collimated when the beam strikes the reflective facet.
The collimating lens 24 and the cross-scan cylindrical lens 26 are the only optical elements in the pre-polygon mirror optical section 12.
The polygon mirror 14 is rotated around its axis of rotation by a conventional motor (not shown), known to those of ordinary skill in the art. The polygon mirror is typically mounted on grease or air bearings (also not shown).
The beam 22 reflected from the facet 16 is still collimated in the scan plane and is now diverging in the cross-scan plane. After reflection from the reflective facet, the beam then passes through a f-theta scan lenses 28 consisting of a negative plano-spherical lens 30 and a positive plano-spherical lens 32. This f-theta scan lenses configuration has sufficient barrel distortion to produce a linear scan beam which then passes through a cross-scan cylindrical lens 34.
The lens 34 may be cylindrical in the cross-scan plane and plano in the scan plane. The cross-scan cylindrical lens will flatten the cross-scan field curvature of the beam from the f-theta lens while the f-theta scan lens together with the cross-scan cylinder lens produces a linear, flat-field scan beam. The f-theta lenses 28 are designed with the cross-scan cylindrical lens 34 because the cross-scan cylindrical lens may contribute a small, but non-negligible, amount of distortion, especially at large scan angles.
After passing through the cross-scan cylindrical lens 34, the beam is then reflected off a cylindrical wobble correction mirror 36 to a scan line 38.
The post-rotating scanning element optical section 18 consists of the f-theta scan lenses 28, the cross-scan cylindrical lens 34 and the cylindrical wobble correction mirror 36.
Optical scanner performance is determined by the physical limitations on the speed at which the polygon mirror is rotated, by the angular deflection of the laser beam achieved by reflection from a facet from the rotating polygon mirror, the size of the facets, and the width of the beam being scanned where it is incident on the rotating polygon mirror.
One method for increasing scanning speeds is the use of angle doubling with small sized polygon mirror assemblies. For an "F-THETA" scan lens, commonly employed in optical scanners, the scanned distance on the photoreceptor is the product of the scan angle (THETA) and the effective focal length (F). Whenever the scan angle can be increased, the effective focal length can be decreased. A decrease in the effective focal length brings two primary advantages. First, the smaller focal length translates directly into a smaller physical casting, or base upon which the optical scanning system components are mounted. Lens elements, mirrors and all other components can be smaller. The result is a smaller, lighter, less costly product. Second, the shorter focal length requires a smaller beam at the rotating polygon, further reducing the sizes of the optical and mechanical components of the optical scanning system.
A further advantage resulting from scan angle doubling is any given scan distance along the photoreceptor can be achieved with only half the polygon mirror angular rotation. By this means, the polygon mirror speed of rotation is significantly reduced, allowing lighter, smaller and less costly motor bearings as well as a longer bearing lifetime and better overall performance.
Scan angle doubling devices are known in the art and have been described as in U.S. Pat. No. 3,973,826 by Lobb which describes a device for passive facet tracking and angle doubling.
Lobb describes two scan angle doubling configurations. The first comprises a rotating mirror which reflects light into a static optical system. The static optical system reflects the received light back onto the rotating mirror. The static optical system is comprised of single system consisting of a roof prism and a field lens or a plurality of static optical systems arranged in an arc in the scanning area, each system comprising a roof prism and a field lens.
In the Lobb patent, the beam is not collimated at the scanner facet in the scanning plane, thus any variation in radius between the facets will translate into scanning errors on the scanning plane. In a laser printing application, these scanning errors show up as pixel placement errors visible on a printed page. When the scanned beam is collimated, in the scan plane at the scanner facet, polygon manufacturing tolerances can be relaxed with resultant cost savings.
In the Lobb patent, the beam is not focused on the scanner facet in the cross-scan plane. As a consequence, pyramidal errors in the scanner facet and bearing wobble will result in variable spacing between scan lines. In a laser printing application, these errors show up on the printed page as differences in spacing between the printed lines. Even very small differences are apparent, producing unacceptable output quality. When the beam is focused on the scanner facet in the cross-scan plane, pyramidal errors may be optically removed by focussing the beam from the facet onto the scan line. Again, polygon mirror manufacturing tolerances can be relaxed with resultant cost savings and no loss in print quality.
The propagation of a light beam can be changed by three basic means: reflection by a mirror, refraction by a lens and diffraction by a grating. Optical systems traditionally rely on reflection and refraction to achieve the desired optical transformation. Optical design, based on mirror and lens elements, is a well-established and refined process. Until recently, the problems with diffraction and fabricating high efficiency diffractive elements have made diffractive elements unfeasible components of optical systems.
The diffractive process does not simply redirect a light beam. Diffraction, unlike refraction and reflection, splits a light beam into many beams--each of which is redirected at a different angle or order. The percentage of the incident light redirected by the desired angle is referred to as the diffraction efficiency. The diffraction efficiency of a diffractive element is determined by the element's surface profile. If the light that is not redirected by the desired angle is substantial, the result will be an intolerable amount of scatter in the image or output plane of the optical system.
Theoretically, diffractive phase elements can achieve 100 percent diffraction efficiency. To achieve this efficiency, however, a continuous phase profile is necessary. The theoretical diffraction efficiency of this surface profile is also relatively sensitive to a change in wavelength. By contrast, refractive and reflective elements are relatively insensitive to changes in wavelength. The technology for producing high quality, high efficiency, continuous phase profiles does not presently exist.
A compromise that results in a relatively high diffraction efficiency and ease of fabrication is a multi-level phase grating. The larger the number of discrete phase levels, the better the approximation of the continuous phase function. These multi-level phase profiles can be fabricated using standard semiconductor integrated circuit fabrication techniques.
As disclosed in Binary Optics Technology: The Theory and Design of Multi-level Diffractive Optical Elements by G.J. Swanson of the Lincoln Laboratory at the Massachusetts Institute of Technology, (Technical Report 854, Aug. 14, 1989) and the resulting U.S. Pat. No. 4,895,790, a fabrication process starts with a mathematical phase description of a diffractive phase profile and results in a fabricated multi-level diffractive surface. The first step is to take the mathematical phase expression and generate from it a set of masks that contain the phase profile information. The second step is to transfer the phase profile information from the masks into the surface of the element specified by the optical element design.
The first step involved in fabricating the multi-level element is to mathematically describe the ideal diffractive phase profile that is to be approximated in a multi-level fashion. The next step in the fabrication process is to create a set of lithographic masks which are produced by standard pattern generators used in the integrated circuit industry.
A substrate of the desired material, such as Ge, ZnSe, Si, and SiO.sub.2, is coated with a thin layer of photoresist. A first lithographic mask is then placed in intimate contact with the substrate and illuminated from above with an ultraviolet exposure lamp. Alternately, pattern generators, either optical or electron beam, can expose the thin layer of photoresist. The photoresist is developed, washing away the exposed resist and leaving the binary grating pattern in the remaining photoresist. This photoresist will act as an etch stop.
The most reliable and accurate way to etch many optical materials is to use reactive ion etching. The process of reactive ion etching anisotropically etches material at very repeatable rates. The desired etch depth can be obtained very accurately. The anisotropic nature of the process assures a vertical etch, resulting in a true binary surface relief profile. Once the substrate has been reactively ion etched to the desired depth, the remaining photoresist is stripped away, leaving a binary phase surface relief grating.
The process may be repeated using a second lithographic mask having half the period of the first mask. The binary phase element is recoated with photoresist and exposed using the second lithographic mask which has half the period of the first mask. After developing and washing away the exposed photoresist, the substrate is reactively ion etched to a depth half that of the first etch. Removal of the remaining photoresist results in a 4 level approximation to the desired profile. The process may be repeated a third and fourth time with lithographic masks having periods of one-quarter and one-eighth that of the first mask, and etching the substrates to depths of one-quarter and one-eighth that of the first etch. The successive etches result in elements having 8 and 16 phase levels. More masks than four might be used, however, fabrication errors tend to predominate as more masks are used.
This process is repeated to produce a multilevel phase relief structure in the substrate. The result is a discrete, computer-generated structure approximating the original idealized diffractive surface. For each additional mask used in the fabrication process, the number of discrete phase levels is doubled, hence the name "binary" optical element or, more precisely, a binary diffractive optical element.
After only four processing iterations, a 16 phase level approximation to the continuous case can be obtained. The process can be carried out in parallel, producing many elements simultaneously, in a cost-effective manner.
A 16 phase level structure achieves 99 percent diffraction efficiency. The residual 1 percent of the light is diffracted into higher orders and manifests itself as scatter. In many optical systems, this is a tolerable amount of scatter. The fabrication of the 16 phase level structure is relatively efficient due to the fact that only four processing iterations are required to produce the element.
After the first etching step, the second and subsequent lithographic masks have to be accurately aligned to the existing pattern on the substrate. Alignment is accomplished using another tool standard to the integrated circuit industry, a mask aligner.
As noted, the photoresist on the substrate can be exposed with an electron-beam pattern generator. The e-beam direct-write process eliminates masks and their corresponding alignment and exposure problems. Binary optics have also been reproduced using epoxy casting, solgel casting, embossing, injection molding and holographic reproduction.
Binary optical elements have a number of advantages over conventional optics. Because they are computer-generated, these elements can perform more generalized wavefront shaping than conventional lenses or mirrors. Elements need only be mathematically defined: no reference surface is necessary. Therefore, wildly asymmetric binary optics are able to correct aberrations in complex optical systems, and elements can be made wavelength-sensitive for special laser systems.
The diffractive optical elements are generally thinner, lighter and can correct for many types of aberrations and distortions. It is possible to approximate a continuous phase profile with a stepwise profile of discrete phase levels.
Binary diffractive optical elements are typically transmissive optical elements. However, binary diffractive optical elements can be reflective optical elements.
The prior art reflective binary diffractive optical element 40 (as taught in U.S. Pat. No. 4,846,552, herein incorporated by reference) of FIG. 2, of a blazed grating, has a surface relief phase grating structure 42 on a substrate 44. The individual blazed gratings of the surface relief structure 42 of the phase grating 40 have a grating period P, and a grating depth H. The surface relief grating 42 itself is formed by a photoresist 46 deposited upon the substrate 44 and a reflective coating 48 deposited upon the photoresist 46.
In this example, the diffraction orders of a beam reflected from a reflective binary diffractive optical element can be made to disappear, except for the first and zeroth order beams into the substrate material. The zeroth order beam can be suppressed independently by control of the relief depth of the binary diffractive surface relief pattern. These suppressed or evanescent orders will redistribute most of their energy into the remaining propagating and radiative orders. These gratings of the reflective binary diffractive optical element behave in a blaze-like or single diffraction order manner. First order diffraction efficiencies of nearly 100 percent can be achieved by proper calculation and selection of the period, width and depth of the grating structure 42.
By way of contrast, transmissive binary diffractive optical elements have typical grating depths of about .lambda./(n-1), n being the refractive index, while reflective diffractive optical elements have typical grating depths of about .lambda./2.
There are, however, practical constraints on the use of a multi-faceted rotating polygon mirror as the scanning element in a raster output scanning system.
One limitation on the speed of a raster scanning system is the maximum polygon mirror rotation speed. It can be appreciated that high quality images require precision placement of the raster scan lines as well as exact timing to define the location of each picture element or pixel along each scan. In a conventional polygon mirror scanner, this precision is achieved by holding very close mechanical tolerances on the polygon mirror geometry and on the rotational bearings supporting the polygon mirror and drive motor. Experience has shown that beyond about 20,000 RPM, precision ball bearings with the required closeness of fit have limited life and are impractical in many scanner applications. As a result, alternatives such as air bearings are sometimes used, but these represent a substantial increase in engineering complexity and maintenance, and hence cost.
The high speed necessary to rotate the polygon mirror can cause deformation of the facets due to centrifugal force with a resulting degradation of the light beam reflected from the deformed facet.
A polygon is not the ideal geometric figure to rotate. The edges of the polygon facets cause drag as they rotate mandating a corresponding increased motor speed and power. Additionally, a rotating polygon is acoustically noisy which serves to degrade an office environment.
The air turbulence of the rotating polygon with its facet edges increases the possibility of contamination of the mirrored facets, increases the load on the bearings, and increases the chance of wobble of the polygon and misalignment of the mirror facets with the light beams.
Multi-faceted polygon mirrors are precision optical elements and as such are expensive and time-consuming to mass produce in quality and quantity.
The present invention uses anamorphic optics to collimate the beam in the scan plane at the scanning facet and to focus the beam in the cross-scan plane at the scanning facet so that errors produced by radial and pyramidal variations of the scanning facets may be substantially reduced or easily corrected to provide for improved scanning.
It is an object of this invention to provide a scan angle doubling optical system with a reflective binary diffractive optical element used as the rotating scanning element.